Its all opinions rightly or wrong. Some are based on having rose tinted glasses on. However, a dives a dive, that was a dive if your not wearing the glasses
A dive is where the player fakes contact and dives to the floor in an attempt to fool the ref.
In this case, Mane has felt contact, he has been impeded by contact, and the only debate is whether he then went to the floor to win the penalty (which was due) or whether he was knocked to the floor by the contact.
Either way it is a penalty.
The question is, would the ref have called it if Mane carried on and then messed up his pass to Lallana because of the foul? Possibly not. By going down, Mane forced the ref to make a decision on the foul.